Non-linear resistances



July 26, 1955 J. sucHl-:T

NONLINEAR RESISTANCES 2 Sheets-Sheet 1 Filed March 25, 1953 mvervron: @cm/ss Suena-r July ze, 1955 J. SUCHET NoN-LINEAR REsIsTANcEs Filed March 25, 1955 2 Sheets-Sheet 2 l w K A v i IrlvEn'To/: TAcQuEs SucHET BY Z,'?l4,096 Patented July 26, i955 Free N ON -LINEAR RESISTANCES Jacques Suchet, Paris, France, assigner to Compagnie Generale de Telegraphie Sans Fil, societe anonyme francaise, Paris, France Application March 25, 1953, Serial No. 344,573 Claims priority, application France March 31, 1952 2 Claims. (Cl. 252-516) The present invention relates to new and useful nonlinear resistances and to processes of making the same, and more particularly to non-linear resistances having improved characteristics and facility of manufacture.

Y Certain electrical resistances present interesting properties, particularly the following:

If V is the voltage applied at the terminals of the resistance and I is the current across the resistance, I and V are in the following relationship:

wherein K is aconstant depending upon the conditions of manufacture and the geometric form of the resistance, and 1i is a coefficient dependent upon the conditions of manufacture and .having a value between 0 and l.

Resistances of this type are designated in the art as non-linear resistances. l

Suchresistances were iirst utilized for the limitation of overvoltages on electrical lines utilized for the transportation of electrical energy at high tension. The use `of these resistances were later extended to the limitation and regulation of overvoltages of lower Value and for other uses such as the protection of relay contacts against sparks due to self-induced breaking current and still further to the utilization in electronic circuits for ordinary voltages, that is to say in the order of several hundreds of volts.

The known non-linear resistances have been prepared wherein the constant K ordinarily varies between 100- i000 and n between 0.2-0.3. p

It yhas become more and more apparent as the application of these resistances developed and expanded that it would be of great interest if resistances could be prepared following the law V=KIn with the constant K having a low value. In other words, it would be of interest to form resistances which would allow passage of considerable current when a low voltage is applied at the terminals thereof, namely resistances wherein the constant K is between 10-100.

Until the present invention non-linear resistances have been made in the form of discs having a diameter of 25 mm. wherein K1=1000 and n=0.25. These resistances allow passage of l ampere for an applied Voltage of 1000. volts. Let us assume that it is desired to allow 'passage of l ampere for an applied voltage of only 100 volts, which is accomplished by putting N of these resistance elements in parallel. The total assembly would be equivalent to a resistance having the characteristics of K2: 100 and n=0.25.

For a given voltage V, such assembly would allow passage of a current NI. The problem which is posed is the choice of N such that V=K2(NI)", the voltagecurrent equation of the assembly. Since V=K1I", the equation referring to each of the individual elements of the assembly, it follows that N must satisfy the following equation:

in other words By replacing the above numerical values for K1, Kg and n, the equation becomes as follows:

lt would therefore require 10,000 of these resistances in order to obtain the desired results. It may therefore be seen that it would be extremely valuable to be able to produce non-linear resistances having a low coeicient K (between 10-100) even if it is necessary to increase the value of the coecient n in order to achieve such resistances.

It has been attempted to manufacture non-linear resistances having a coeicient K between 10-100 by the utilization of mixtures of powdered carborundum (silicon carbide) in pure state with variable quantities of c011- ductor materials which are themselves not linear, i. e. carbon and iron, and to form an agglomerate of the` mixture.

However, these mixtures have resulted in considerable difficulties of manufacture. In effect, the conditions of manufacture, such as duration of sintering of the miX- ture, temperature or composition of the atmosphere, had to be varied considerably with the concentration ofcarborundum in the mixture. Moreover, the improvement in the value of the factor K is thus obtained only at the price ofserious increase in the value of the factor n, which approaches 1 so close as to limit the utilization of these resistances. I

It is therefore `an object of the present invention to provide for non-linear resistances wherein the value of the constant K is low without a corresponding too great an increase in the value of n.

The present invention comprises the production of nonlinear resistances by the shaping and sintering together of a mass of particles of silicon carbide `and between Ztl-40% of a semi-conductor oxide which is partially reduced and which has a deficiency of oxygen in the crystalline form thereof, the semi-conductor oxide'being selected from the group of incompletely oxidized oxides of vanadium, zinc and titanium. A preferred semi-conductor oxide is TisOs. y

lt is another object of thefpresent invention to provide a process of forming non-linear resistances of the above type.

lt is a further object of the present invention-to provide a process of forming non-linear resistances of the above type wherein the value of the constant K can be varied without the necessity of varying the condition of manufacture of the resistance.

It is still a further object of the present invention to provide a process of manufacturing non-linear resistances whereby set conditions of manufacture can be set up and wherein the value of K can bevaried in the final resistance without varying the set conditions of manufacture, but merely by changing the proportions of the ingredients.

It is still another object of the present invention to provide non-linear resistances which have a functioning temperature not much greater than the ambient temperature and the characteristics of which vary only slightly with variation in temperature.

Other objects and advantages of the present invention will be apparent from further reading of the specification and of the appended claims.

With the above objects in view, the present invention mainly co-mprises in a process of producing a non-linear resistance following during operation the equation V=KIn in which equation V is the voltage applied at the teminals of the resistance,

I is the current across the resistance,

K is a constant depending upon the conditions of production of said resistance and the geometric form thereof, and

n is a coefficient -depending upon the conditions of production of said resistance, the step of sintering to a predetermined geornetric form a mixture of finely divided particles of silicon carbide and a powdered semi-conductor oxide mixed in such proportion that for the finished resistance the value of the constant K is between -100 and the value of the coefficient n is greater than 0 and not greater than 0.4.

It may be seen therefore that according to the present invention the basic mixture from which the non-linear resistances are manufactured is a binary mixture consisting partly of powdered carborundum and partly of a semi-conductor oxide which itself has the phenomenon of being non-linear, the factor K for a given geometric form of said semi-conductor being clearly less than the value K for the same geometric form of pure carborundum.

It is preferred according to the present invention to utilize as semi-conductor, saturated oxides which can have a deficiency of oxygen in the crystalline form therey of, such as oxides of vanadium, zinc and titanium after reduction. In other words, these oxides are still further oxidizable and may therefore be considered as being partially reduced or incompletely oxidized. These oxides may be formed by partially reducing the completely oxidized metal.

These oxides have a lack of oxygen ions in the crystalline lattice, i. e. ZnOoss. Very few oxides have this property of having a deficiency of oxygen ions in the crystalline lattice of the compound. Incompletely oxidized oxides of vanadium, zinc and titanium have been found most suitable according to the present invention. It has been found most advantageous to utilize a titanium Vsub-oxide, namely TisOs. It has been found preferable vto utilize amounts of -40% by weight of a semi-conductor oxide in generally no more than by weight.

The non-linearity of these oxides is due to a superficial adsorption of oxygen after the heat treatment and terminating in the formation ori each grain of an insulating coating of very slight thickness.

The process of forming the non-linear resistances according to the present invention may be carried out as follows:

Powdered carborundum is finely ground in a manner such that the maximum grain size of the carborundum does not exceed microns.

`With this powder is mixed a powdered saturated semiconductor oxide, previously reduced by hydrogen, the dimensions of the grains of this powdered semi-conductor oxide not exceeding 150 microns, the proportion of semi-conductor oxide to c'arborundum being as above specified. The mixture is malaxated in a rotating container by means of porcelain balls for a sufiicient time period (about 100 hours).

To the thus obtained mass is then added an organic binder such as urea at a sufiiciently high temperature, the amount of said binder being at least 10% by weight of the total mixture. The entire mass is then introduced into a special malaxator after which the following op- Verations are performed:

VThe mass is compressed and shaped with the aid of automatic devices utilizing a pressure well below 1 ton/cm2- In the course of this operation the volume of the mixture need not be decreased to less than one-half the original volume, this being possible due to the presence of the organic binder.

The shaped mass is then sintered in a reducing atmosphere at a temperature sufficiently high to burn-out the organic binder and to sinter together the particles of silicon carbide and semi-conductor oxide. The temperature utilized is generally between 100G-12007 C., the sintering being carried out in a controlled atmosphere. The resistances then undergo a known metallization treatment. The non-linear resistances thus obtained are generally in the form of discs.

It should be noted that the constant K for resistances of predetermined form depends upon the proportion of semi-conductor oxide.

A considerable advantage of the present invention results from the variation of the constant K with the proportion of the semi-conductor oxide. With carborundum alone, in order to vary the value of K, it is necessary to vary the conditions of manufacture, for example the duration of sintering, the temperature or the composition of the atmosphere. The process of the present invention, on the other hand, presents, contrary to the known processes, the possibility of allowing the operating conditions to remain constant and changing of the value of K simply by varying the proportions of carborundum and the semi-conductor oxide in the mixture which is submitted to the sintering operation. Moreover, the addition of such semi-conductor oxide, according to the present invention permits the obtaining of values of K very much lower than could be obtained utilizing carborundum alone, in fact, values which are ten times lower.

35% Vtitanium oxide, for example, after reduction by hydrogen to a composition approximating Ti3O5 and prolonged grinding permits the obtention of a resistance wherein K=30 and n=0.4, as compared to the same resistance which if made from 100% carborundum would have values of K=300 and n=0.15.

The accompanying drawings are given for a better understanding of the objects and advantages of the present invention, the drawings consisting of:

Fig. l which illustrates the variations of the constants .of non-linear resistances with changes in concentration of TisOs and SiC; and

Fig. 2 which indicates by a graph the voltage to be applied to a resistance having specified values of the constants in order to achieve a given intensity of current.

The variation of the constants K and n with the concentration of finely divided mixtures of titanium suboxide is indicated in Fig. l for non-linear resistances in the form of thin discs of a given diameter and formed under given conditions.

Fig. 2 indicates the characteristics of these resistances for various values of K.

In Fig. 1 the proportion of carborundum (silicon carbide) is plotted as abscissa and the values of K and n of the final resistance obtained are plotted as ordinates. It may be seen that the value of K diminishes as rapidly with an increase in the proportion of carborundum as the value of n is increased to attain a value of n equal to 0.4 for a proportion of silicon carbide in the order of about Mixtures comprising concentrations of titanium suboxide (TiaO5) greater than 35% result in still lower values for K, but the value of the coefiicient n approaches too close to l for the best utilization of such mixtures for the production of non-linear resistances. There appears, on the other hand, to be an extreme sensitivity to Vthermal treatments which is harmful to precision manufacture.

In Fig. 2 is plotted the determined characteristics K and n for various compositions, whereby the voltage to be applied at the terminals of the resistance in order to achieve a given intensity of current, may be determined.

If non-linear resistances having a carborundum base and being in the form of discs having a diameter of 25 mm. and a thickness of 1 mm. are desired with a constant K=150, the same can be obtained by the addition of small quantities of graphite to the carborundum; however, this results in the coefficient n equal to about 0.33.

The addition according to the present invention on the other hand of titanium sub-oxide allows for achieving of the same Value of K for the same geometric form, but wherein the coel'lcient n is equal to 0.25.

The advantage represented by the possibility of determining one time for all times the conditions of sintering and the composition of the atmosphere during sintering is a great advantage in the manufacture of these non-linear resistances and approaches automatic manufacture of the same, in addition to advantages of less pressure for compression, high compression of the powders and low sintering temperature. Together these advantages result in the realization of an automatic or semiautomatic manufacture of the classical type in powder metallurgy (compression-simering-metallization) with presses and furnaces of current design.

The following example is given as illustrative of a preferred method of proceeding according to the present invention, the scope of said invention not, however, being limited to the specific example.

Example Per cent Silicon carbide -60 Titanium oxide (reduced) -20 Lead silicate -20 Balls of porcelain are placed in the mixture and the 6 jar is closed and rotated for several days. The contents are then removed and mixed at 140 C. with 15% urea in a special malaxator and then stored.

The powder is then molded by means of a press utilizing a low pressure into the form of discs of 25 mm. diameter. The shaped discs are then sintered in a reducing atmdsphere at a temperature of about 1000 C. The discs are then recoated on both faces with powdered silver after which the connections are soldered and a protecting varnish is applied to the element, which is then submitted to an electrical stabilization treatment.

The characteristics of such elements are approximately:

K=50 and n=0.35

Without further analysis, Ithe foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

l. A non-linear resistance comprising a shaped, sintered mass of nely divided particles of silicon carbide and between 20-40% of a semi-conductor oxide being partially reduced and having a deficiency of oxygen in the crystalline form thereof and being selected from the group consisting of incompletely oxidized oxides of vanadium, zinc and titanium.

2. A non-linear resistance comprising a shaped, sintered mass of finely divided particles of silicon carbide and between 20-40% of Ti3O5.

No references cited. 

1. A NON-LINEAR RESISTANCE COMPRISING A SHAPED, SINTARED MASS OF FINELY DIVIDED PARTICLES OF SILICON CARBIDE AND BETWEEN 20-40% OD A SEMI-CONDUCTOR OXIDE BEING PARTIALLY REDUCED AND HAVING A DEFICIENCY OF OXYGEN IN THE CRYSTALLING FORM THEREOF AND BEING SELECTED FROM THE GROUP CONSISTING OF INCOMPLETELY OXIDIZED OXIDES OF VANADIUM, ZINC AND TITANIUM. 